Chinese translation / 中文本地化翻译 about fpm-docs

[henilp105]

Thanks @zoziha , I have patched this in PR #121 .

[zoziha]

@henilp105 @awvwgk

I noticed that the Chinese translation website "https://fpm.fortran-lang.org/cn/index.html" is not accessible, as well as "https://fpm.fortran-lang.org/zh_cn/index.html", check the source code:

fpm-docs/pages/_templates/sbt-sidebar-footer.html

Lines 5 to 10 in a340970

	{%- for lang in ('zh_CN', 'cs', 'de', 'en', 'es', 'fr', 'ja', 'nl', 'pt', 'ru') %}
	{%- if lang == language %}
	<b>{{lang.lower()[-2:]}}</b>
	{%- else %}
	<a href="{{"https://fpm.fortran-lang.org/" + ('' if lang == 'en' else lang.lower()[-2:]) + "/" + pagename +".html" }}">{{lang.lower()[-2:]}}</a>
	{%- endif %}

I found that the use of lang.lower()[-2:] here means that zh_CN is replaced with cn.
Actually only "https://fpm.fortran-lang.org/zh_CN/index.html" is accessible.